In recent years, the growth in the information throughput of various types of electronic equipment has been accompanied by rapid advances in packaging technology, including higher integration of the semiconductor devices within the equipment, higher wiring density, and multi layer construction. To increase the signal transmission rate and reduce loss during signal transmission, there exists a desire for insulating materials in printed wiring boards and the like that are used in various electronic equipment to have a low dielectric constant and a low dielectric loss tangent.
Because polyphenylene ethers (PPE) have excellent dielectric properties such as dielectric constant and loss tangent even in the MHz to GHz high-frequency bands (high-frequency regions), they are effectively used in insulating materials for printed wiring boards and the like in electronic equipment which utilizes high-frequency bands. However, because high-molecular-weight PPEs generally have a high melting point, they tend to have a high viscosity and a low flowability. In addition, when such PPEs are used to form prepregs for use in the manufacture of multilayer printed wiring boards and the like and when printed wiring boards are manufactured using the prepregs thus formed, moldability problems have arisen; namely, molding defects such as voids arise during multilayer molding, making it difficult to obtain printed wiring boards having a high reliability. One known technique for resolving such problems involves lowering the molecular weight of the PPE by subjecting high-molecular-weight PPE to a redistribution reaction within a solvent and in the presence of a phenol compound and a radical initiator so as to give rise to molecular cleavage. However, when PPE is rendered to a low molecular weight, curing is inadequate and the heat resistance and other properties of the cured product tend to decline.
Hence, in order to increase the heat resistance and other properties of the cured product, as described in Patent Literature 1 below, the use of a thermoset resin such as an epoxy resin in combination with PPE has been reported. Specifically, Patent Literature 1 describes a resin composition which is characterized by including the reaction product of a polyphenylene ether having a number-average molecular weight of from 800 to 2,000 and an average of 1.5 to 2 hydroxyl groups per molecule with an epoxy resin having a small number of epoxy groups, namely, an average of 2.3 or fewer epoxy groups per molecule, and also a thermoset resin.
The art in Patent Literature 1 indicates that the heat resistance and the adhesion can be enhanced while retaining the excellent dielectric properties of PPE. However, in recent years the standards required of printed wiring boards and the like have risen further, and so materials endowed with even better properties are now desired in order to meet the demand for higher performance. For example, to enhance the reliability and heat resistance in high multilayer printed wiring boards, further improvements are desired not only in the high glass transition temperature (Tg) and heat resistance of cured resin materials, but also in the bond strength (the adhesive strength between layers or with copper foil in a copper-clad laminate or the like).
The invention has been made in view of the above circumstances, and its object is to provide a resin composition which has an even higher level of bond strength while retaining the excellent dielectric properties of PPE and also the heat resistance and other properties of cured products thereof. Further objects of the invention are to provide a prepreg that uses such a resin composition, a metal-clad laminate that uses such a prepreg, and a printed wiring board that uses such a metal-clad laminate.